In recent years, OCT that forms images expressing surface and internal morphologies of an object by using light beam from laser light source etc. has attracted attention. OCT is noninvasive to human bodies unlike X-ray CT and is therefore expected to be utilized in medical and biological fields in particular. For example, apparatuses that form images of fundus, cornea etc. are in a practical stage in ophthalmology.
An apparatus disclosed in Patent Document 1 uses a technique so-called “Fourier Domain OCT.” Specifically, the apparatus irradiates low-coherence light beam to an object, superposes reflected light thereof and reference light to generate interference light, acquires spectral intensity distribution of the interference light and executes Fourier transform on it, thereby imaging morphology of the object along a depth direction (z-direction). Further, the apparatus is provided with a galvano mirror for scanning light beam (measurement light) in one direction (x-direction) perpendicular to the z-direction and forms an image of a desired measurement target region of the object. An image formed by this apparatus is a two-dimensional cross-sectional image in the depth direction (z-direction) along the scanning direction (x-direction) of the light beam. The technique of this type is also called Spectral Domain.
Patent Document 2 discloses a technique of scanning measurement light in horizontal and vertical directions (x-direction and y-direction) to form multiple two-dimensional cross-sectional images along the horizontal direction and of acquiring and imaging three-dimensional cross-sectional information of a measured area based on the cross-sectional images. examples of such three-dimensional imaging include a method of arranging and displaying cross-sectional images along the vertical direction (referred to as stack data etc.), a method of executing rendering on volume data (voxel data) based on stack data to form a three-dimensional image.
Patent Documents 3 and 4 disclose other types of OCT. Patent Document 3 describes an OCT apparatus that images morphology of an object by scanning wavelength of light irradiated to an object (wavelength sweeping), detecting interference light obtained by superposing reflected lights of respective wavelengths on reference light to acquire spectral intensity distribution and executing Fourier transform on it. Such an OCT is called Swept Source type. The Swept Source type is a kind of the Fourier Domain type.
Patent Document 4 describes an OCT apparatus that irradiates light having a certain beam diameter to an object and analyzes components of interference light obtained by superposing reflected light thereof and reference light, thereby forming an image of the object in a cross-section orthogonal to travelling direction of the light. Such an OCT apparatus is called full-field type or en-face type.
Patent Document 5 discloses an application of OCT to ophthalmology. Before OCT was applied, retinal cameras, slit lamp microscopes, scanning laser ophthalmoscopes (SLO) etc. were used for observing an eye (see Patent Documents 6, 7 and 8 for example). A retinal camera photographs a fundus by projecting illumination light on an eye and receiving reflected light from the fundus. A slit lamp microscope obtains a cross-sectional image of a cornea by cutting off light section of the cornea by using slit light. An SLO images morphology of retinal surface by scanning a fundus with laser light and detecting reflected light with a highly sensitive imaging element such as a photomultiplier.
As described above, OCT is superior relative to retinal cameras etc. in that high-definition image may be obtained, further in that cross-sectional image and three-dimensional image may be obtained, etc.
Thus, ophthalmologic imaging apparatuses using OCT may be used for observation of various sites of an eye and is capable of acquiring high-definition images; therefore, OCT has been applied to diagnoses of various ophthalmologic disorders. Now, ophthalmologic imaging apparatuses capable of performing OCT measurement of both fundus and anterior eye part are sometimes used for observing various sites of eyes. An Attachment (adopter or optical unit) for changing focus position of measurement light from fundus to anterior eye part is selectively applied to such an ophthalmologic imaging apparatus (see Patent Document 9). This attachment includes a lens having predetermined refractive power.